1. Field of the Invention
This invention relates to the fabrication of semiconductor devices, and more specifically, to methods for fabricating high quality oxides for semiconductor devices.
2. Description of the Related Art
High quality oxides are important in the fabrication of semiconductor devices. This is particularly true in such devices as electrically erasable programmable read only memories (EEPROMs) and gate oxides of MOS transistors where the oxides function as dielectrics and are key to electrical performance of the device. The thin tunnel oxides of EEPROM devices typically have thicknesses well under 100 .ANG.. High quality dielectrics are needed in such devices to achieve satisfactory device performance both in terms of speed and longevity.
It has been found that the presence of nitrogen in the oxide significantly improves the tunnel oxides of EEPROM devices as well as gate oxides of MOS transistors. The presence of nitrogen in the oxide layer, typically a silicon dioxide film, significantly improves the breakdown characteristics of the film. The role of the nitrogen in improving the oxides has been postulated to relax the Si--O bonds by forming Si--N bonds or N--O bonds. See H. Fukada et al. IEEE Elect. Dev. Letters, vol 12, no. 11, 1991 and A. T. Wu et al., Appl. Physics. Lett., vol. 55, 1989. The formation of Si--N or N--O bonds enhances the bond strength and reduces the interface trap density.
MOS capacitors with N.sub.2 O nitrided oxides show extremely tight Time Dependent Dielectric Breakdown (TDDB) distributions. The improvement in TDDB persists even after complete processing and is observed for various structures such as capacitors on p-substrate, capacitors on n+ implanted regions, and surface capacitors.
It has recently been discovered that the use of the source gas N.sub.2 O as a source of nitrogen following thin gate oxide growth is an effective source of nitrogen under specific temperature conditions and thus can significantly enhance the dielectric qualities of the oxide film. The mechanism is believed to work by the nitrogen incorporating itself at the Si/SiO.sub.2 interface, replacing the hydrogen that has attached itself to the dangling bonds at that interface. The bonding strength of nitrogen is much higher than that of hydrogen making for a much more stable film under thermal, electric field or radiation stress.
In tunnel oxides, breakdown occurs because of the trapping of charge in the oxide, thereby gradually raising the electric field across the oxide until the oxide can no longer withstand the induced voltage. Higher quality oxides trap less charge over time and will therefore take longer to break down.
It is believed that the improvement in time to breakdown for both tunnel oxides and gate oxides is due to the charge stability in the Si/SiO.sub.2 interface, the poly-silicon/SiO.sub.2 and throughout the gate oxide and tunnel oxide region afforded by the presence of nitrogen and the tunnel in gate oxides.
The use of the source gas N.sub.2 O following thin gate oxide growth is described in U.S. Pat. No. 5,296,411 which is assigned to the same assignee, and which is incorporated herein by reference.
In the U.S. Pat. No. 5,296,411, a rapid thermal anneal is performed in an N.sub.2 O ambient environment following formation of a thin oxide layer. In order for the N.sub.2 O anneal to be effective, when utilized in, e.g., a diffusion tube or an RTA system, a temperature of approximately 1,050.degree. C. has been utilized previously. It has been observed that just adding nitrogen as a source gas will not provide the same results. Thus, it was key to use N.sub.2 O as a source gas and to decompose N.sub.2 O with a sufficiently high temperature in order for the improvement due to the presence of nitrogen in the Si/SiO.sub.2 interface to be observed. It is noted that an O.sub.2 +N.sub.2 mixture will not produce the required improvement of the gate oxide (or tunnel oxide). Further, having O.sub.2 present during the anneal step provides additional unwanted oxidation. An anneal step in an N.sub.2 O ambient as described in the U.S. Pat. No. 5,296,411, forms approximately another 15 .ANG. to the oxide layer.
The use of the high temperatures needed for N.sub.2 O breakdown can cause increased stress on the semiconductor wafer as well as increasing the dopant diffusion. Additionally, even when N.sub.2 O is broken down using relatively high temperatures, the amount of NO present is only approximately 5%.
Thus, despite the quality improvement shown in oxides due to the use of an N.sub.2 O anneal, improvements which further enhance oxide quality and a process that provides this improvement at low temperatures is desirable.